Conventional shock absorbers are normally of the piston-cylinder type filled with hydraulic fluid and provided with a powerful spring between a first end of the cylinder and an extending end of the piston rod associated with the piston head in the cylinder. The piston head is provided with small bypass passages so that the fluid can move from one side of the piston head to the other upon reciprocating movement of the head resulting from a series of shocks as the vehicle travels along the road. The fluid effectively dampens the vibratory motion to help smooth out the ride.
Normally, shock absorbers of the foregoing type are connected in parallel; that is, across the normally provided coil or leaf type suspension spring for the vehicle. Thus, when the vehicle is travelling on a very smooth road, there will be little action by the shock absorber. On the other hand, when the vehicle rapidly accelerates or decelerates or rounds a curve, relatively large sustained forces are applied to the shock absorber. In the case of the shock absorbers for the rear wheels, under accelerating conditions there will be a large downward load on the shock absorber and it would be desirable in such instances to "stiffen up" the action of the movement of the piston head in the cylinder. On the other hand, when the vehicle is decelerating, the rear shock absorbers will be subject to a relatively light load and to be effective, less "stiffness" would be desirable. It will also be appreciated that when a shock absorber is under a heavy load and the vehicle is passing over a bumpy road so that the piston head in the shock absorber cylinder is moving rapidly back and forth, the piston head will tend to work upwardly towards the top of the shock absorber cylinder and even hit against the cylinder head. This action is prevented or delayed if the pressure and "stiffness" or damping of the piston head can be increased under such heavy loads.
From all of the foregoing, greatly improved action of shock absorbers particularly for motorcycles or racing vehicles could be realized if some means were provided for automatically controlling the fluid pressure and thus the "stiffness" of the shock absorber under certain vehicle operating conditions.